EuiDataGrid add column sorting

[chandlerprall]

Summary

Adds a sorting prop to describe the currently state of sort and a callback function when the state is changed by user interaction. By default the grid does not sort anything itself, but this can be opted into by passing the inMemory prop, demonstrated by a new example in the docs.

[thompsongl]

The initial state has [{ id: 'contributions', direction: 'asc' }] and the toggles correctly show that 'contributions' should be in ascending order, but the column looks to be unsorted.

[myasonik]

If the length > 2 can we add an aria-describedby attribute on the cell?

None of this is defined in the spec so we just need to make up a good experience. This seems like a good starting place to me though I'm open to any other suggestions too.

Pseudo-code simplified example:

const multiSort = sorting.columns.length > 1;
const ariaDescribeBy =  multiSort ? { 'aria-describedby': randomId() } : {};
const sortString = sorting.columns.map(col => `Sort by ${col.name} ${col.direction}`).join(' then ');
}, '');

return (
  <div role="columnheader" {...ariaDescribeBy}>
    Column name
	{multiSort && 
      <EuiScreenReaderOnly id={randomId}>
		{sortString}
      </EuiScreenReaderOnly>
    }
  </div>
)

[myasonik]

The in memory table never gets the top left cell tabIndex set to 0 by default so it's impossible to tab into (without first interacting with it with a mouse).

I tested it with just the memory table rendered, not only 2 tables on one page, so I don't think it's the same focus issues that @snide is hitting in #2230.

[chandlerprall]

The initial state has [{ id: 'contributions', direction: 'asc' }] and the toggles correctly show that 'contributions' should be in ascending order, but the column looks to be unsorted.

@thompsongl currently the in-memory sort is purely text based and ends up as a lexicographical sort. As an example:

['2', '10', '3', '13'].sort() // ["10", "13", "2", "3"]

With the upcoming schema changes the sorting will know what kind of data is being sorted and will be smarter.

[chandlerprall]

Fixed up the sorting after @snide's merge and addressed the tab-into and arrow key navigation @myasonik found.

There's a performance problem caused by having the focus trap on every cell, but I'm leaving that for another PR.

This is ready for another review pass.

[thompsongl]

Possibly something to be addressed in a separate PR:

1. From the in-memory grid, tab into the grid
2. arrow to, say, the 6th row
3. tab to 'Rows per page' and change to '5 rows`

Focus moves to the body, and it's no longer possible to tab into the grid (until pagination is changed back)

[chandlerprall]

@thompsongl good catch! I'm going to come back around to that in another PR, need to clean up how the focused cell is tracked.

[chandlerprall]

As @myasonik is out and no other (sorting-related) issues have presented, I'm merging this into the feature branch to allow other cleanups to begin.